Composite oxygen ion transport membranes have been proposed for a variety of uses that involve the production of essentially pure oxygen by separation of oxygen from an oxygen containing feed through oxygen ion transport through such membrane. For example, each membrane can be used in combustion devices to support oxy-fuel combustion or for partial oxidation reactions involving the production of a synthesis gas.
In such membranes, the oxygen ion transport principally occurs within a dense layer that allows both oxygen ions and electrons transport at elevated temperatures. The oxygen from an oxygen containing feed ionizes on one surface of the membrane and the resultant oxygen ions are driven through the dense layer and emerge on the opposite side thereof to recombine into elemental oxygen. In the recombination, electrons are liberated and are transported back through the membrane to ionize the oxygen.
Such membranes can employ two phases, an ionic phase to conduct the oxygen ions and an electronic phase to conduct the electrons. In order to minimize the resistance of the membrane to the ionic transport, such membranes are made as thin as practical and are supported on porous support layers. The resulting composite oxygen transport membrane can be fabricated as a planar element or as a tube in which the dense layer is situated either on the inside or the outside of the tube.
An example of a composite oxygen ion transport membrane is disclosed in U.S. Pat. No. 5,240,480 that has a dense layer supported on two porous layers. The dense layer can be formed of an ionic conducting phase that contains yttrium stabilized zirconia and an electronic conducting phase that is formed from platinum or another noble metal. The porous layer adjacent to the dense layer is active and is capable of conducting oxygen ions and electrons. The other porous layer can be yttrium stabilized zirconia or calcium-stabilized zirconia.
U.S. Pat. No. 5,478,444 discloses a two-phase material capable of transporting oxygen ions and electrons. The oxygen ion conducting phase can be a metallic cerium oxide incorporating an yttrium stabilizer and a dopant that can be iron or cobalt. The electronic conducting phase is a perovskite that contains lanthanum, strontium, magnesium and cobalt or lanthanum, strontium cobalt and iron.
U.S. Pat. No. 5,306,411 discloses a dual-phase membrane having an ionic conducting phase formed from Sc2O3-stabilized zirconia. The electronic conducting phase can be a perovskite material containing, for example lanthanum, strontium, irons, chromium and cobalt. The resultant dense layer can be supported on an yttria-stabilized zirconia.
U.S. Pat. No. 7,556,676 discloses a dual-phase membrane having an ionic conducting fluorite phase formed of Sc-doped zirconia and an electronic conducting perovskite phase containing lanthanum, strontium, chromium, iron, and a small amount of vanadium. The dense membrane is supported on a thick 3 mol % yttria-stabilized zirconia (3YSZ) substrate. To densify the vanadium-containing perovskite, a reducing atmosphere of hydrogen and nitrogen must be used. The dense membrane also has two optional layers: a porous fuel oxidation layer to reduce the electrochemical resistance for fuel oxidation and a porous layer on the air side to facilitate oxygen reduction to oxygen ions. The main problems with this membrane are its low oxygen flux and fast degradation of oxygen flux during long-term operation. The low flux and fast degradation might be related to the membrane fabrication process in reducing environment under which the perovskite phase may react with zirconia to form an electrically insulating third phase and densification of both fuel oxidation and air reduction layers.
To address these problems, U.S. Pat. No. 8,795,417 B2 discloses a dual-phase oxygen transport membrane consisting of a vanadium-free perovskite phase and Sc-doped zirconia phase supported on a thick 3YSZ substrate. The perovskite phase which contains lanthanum, strontium, chromium, iron and no vanadium is densified by sintering in air at temperatures from 1400 to 1430° C. The sintering process in air eliminates the formation of an electrically insulating third phase and reduces the fabrication cost. A porous fuel oxidation layer is formed from a calcium-containing perovskite and a doped zirconia. The fuel oxidation layer made of calcium-containing perovskites is more refractory and therefore, tends to have a more stable microstructure during high-temperature operation. However, one problem with this oxygen transport membrane is that the 3YSZ porous support after high temperature sintering experiences phase transformation from tetragonal to monoclinic when stored at room temperature in ambient air. The phase transformation is accompanied by about 5% volume increase and results in cracking of the porous support or delamination of the coating from the porous support.
As will be discussed the present invention provides a composite oxygen ion transport membrane that incorporates materials to enable fabrication to be accomplished in a more cost effective manner than in the prior art. Also, the present membrane is more durable than prior art membranes by avoiding the detrimental tetragonal-to-monoclinic phase transformation of the porous support. Furthermore, the materials used in all three active layers are similar in composition so the shrinkage of each layer is closely matched during membrane fabrication, results in minimal residual stress. The current oxygen transport membrane also exhibits improved oxygen flux and reduced degradation of oxygen flux during long term operation due to the inherent properties of the composition of the dense separation layer, fuel oxidation layer, and surface exchange layer and the fabrication process.